U.S. Pat. No. 3,481,952 discloses a class of maleimide compounds which can be used as spin labels for biological molecules to study their motional characteristics by electron paramagnetic resonance (EPR) spectroscopy. With reference to such maleimide spin labels, the widely used compound has been N-(1-oxyl-2,2,6,6-tetramethyl-1-piperdinyl-oxyl)maleimide (.sup.14 N-HMSL). Beth, et al have shown that .sup.14 N-HMSL can be improved as a spin label with respect to its sensitivity and resolution capability by substitution of deuterium for all hydrogens in the heterocyclic ring containing the paramagnetic group. Beth, et al, Chem. Phys. Letts. (1980), 69, 24-28. The specific compound disclosed was N-(1-oxyl-2,2,6,6-tetramethyl-4-piperidinyl)maleimide (.sup.14 N-DMSL).